A ventilator delivers a flow of pressurized gas, such as air and/or a mixture of air and extra (supplemental) oxygen, to the airway of a patient in order to assist in or substitute for the patient's breathing. A ventilator operates cyclically, such that the gas is provided to the patient during an inspiratory phase (corresponding to inhalation) and received from the patient during a subsequent expiratory phase (corresponding to exhalation). In order to provide a mixture of air and extra oxygen, for example, the ventilator receives air through an air pathway and pure oxygen through a separate oxygen pathway, and thus controls respective levels of each gas to obtain the desired mixture, provided to the patient during the inspiratory phase.
Generally, the patient interacts with a ventilator through conduits or “limbs” which conduct gas flow. A single limb ventilator provides a single conduit for inspiratory and expiratory phases, meaning that the patient receives (pressurized) gas from the ventilator during inhalation and discharges gas to the ventilator during exhalation through the same conduit. Typically, the discharged gas is directed through the air pathway of the ventilator. When the inspiratory gas flow includes a mixture of air and oxygen, for example, the expiratory gas flow necessarily includes at least a portion of the extra oxygen, resulting in “oxygen contamination” in the air pathway. Therefore, during the subsequent cycle of the inspiratory gas flow, the gas from the air pathway includes a higher concentration of oxygen than pure air. When the gas from the air pathway is mixed with additional oxygen from the oxygen pathway, the mixed gas provided to the patient has a higher than desired concentration of oxygen.
In contrast, a double limb ventilator avoids oxygen contamination of the air pathway by providing separate conduits for inspiratory and expiratory gas flows. That is, the patient receives (pressurized) gas from the ventilator through a first limb during inhalation, and discharges gas to the ventilator (or outside the ventilator) through a separate second limb to during exhalation. However, the inclusion and maintenance of two separate conduits increases complexity and expense of the ventilator. For example, a valve must be included to direct the inspiratory and expiratory gas flows to the appropriate conduits for proper operation.
In one aspect of the invention, a ventilator includes first and second pathways, a conduit and a controller. The first pathway is configured to supply a first gas and the second pathway is configured to supply a second gas, where the second gas is mixed with the first gas to produce mixed gas having a predetermined percentage of the second gas. The conduit is configured to provide the mixed gas from the first and second pathways to an access port during an inspiratory phase, and to provide discharged gas from the access port to the first pathway during an expiratory phase. The controller is configured to delay supply of the second gas from the second pathway for a delay time in order to maintain the predetermined percentage of the second gas in the mixed gas provided to the access port during a subsequent inspiratory phase.
In another aspect of the invention, a ventilator includes air and oxygen pathways, a blower, a conduit and a controller. The air pathway is configured to supply air to a mixing node. The oxygen pathway is configured to supply oxygen to the mixing node, where the oxygen is mixed with air to obtain a mixed gas having a predetermined percentage of oxygen. The blower is configured to pressurize the mixed gas during an inspiratory phase. The conduit is configured to provide the pressurized mixed gas from the blower to an access port during the inspiratory phase, and to provide discharged gas from the access port to the air pathway during an expiratory phase. The controller is configured to delay supply of the oxygen gas from the oxygen pathway for a delay time in order to maintain the predetermined percentage of the oxygen in the mixed gas during a subsequent inspiratory phase.
In another aspect of the invention, a method is provided for controlling contents of a mixed gas provided to a patient by a single limb ventilator during an inspiratory phase, the mixed gas including a predetermined amount of extra oxygen mixed with air. The method includes measuring flow of a discharged gas through an air pathway during an expiratory phase, the discharged gas including at least a portion of the extra oxygen of the mixed gas; calculating a volume of the discharged gas based on the measured flow; determining a delay time based on the calculated volume; and stopping flow of oxygen from an oxygen pathway, separate from the air pathway, for the determined delay time in order to compensate for the portion of the extra oxygen in the discharged gas during a subsequent inspiratory phase.